The apparatus described in U.S. Pat. No. 4,366,125, which is incorporated herein by reference in its entirety, including any references cited therein, generates a hydrogen peroxide mist by an ultrasonic waves vibrator. The mist adheres to the surface of materials being sterilized and is then irradiated with ultraviolet-ray lamps. U.S. Pat. Nos. 5,878,355 and 6,102,992, each of which is incorporated herein by reference in its entirety, including any references cited therein, disclose a method and device for decontamination of a contaminated process area whereby a fine aerosol of an encapsulant is generated to encapsulate contaminants within a contaminated environment. The aerosol is generated by one or more ultrasonic transducers located below the surface of a reservoir containing a liquid. The output of the transducers is focused to either a point and/or directed toward an area near the surface of the liquid to cause a surface disturbance, which results in the formation of an aerosol from the liquid. The transducers used in these apparatuses are made from lead-zirconate-titanate-four (PZT-4) or other piezoelectric materials. This material is coated with a conductive coating (electrode material) that enables an electrical signal to energize the transducer and causes it to emit high frequency pressure (energy).
While operating these prior art apparatuses and similar apparatuses, it has been found that certain liquids, especially acidic solutions, chemically react with the electrode materials of the transducers that generate the aerosol. The result is a noticeable deterioration of both the transducers and their performance. For example, acidic solutions of hydrogen peroxide and peroxyacetic acid have caused noticeable deterioration of the transducers within minutes of operation.
An attempt was made to prevent transducer degradation by coating the face of the transducers with a thin coating of different materials. None of these efforts have been successful. For example, U.S. Pat. No. 4,109,863, which is incorporated herein by reference in its entirety, including any references cited therein, discloses similar findings. The protective coating on the transducer deteriorated to a point where the transducer failed to be energized. It was initially believed that this deterioration was caused by transducer induced cavitation within the tank; however, the aforementioned coatings were also shown to fail in simple immersion tests, conducted over time in an acidic solution, with unpowered transducers. For example, laboratory work indicated that PZT material coated with an electroless nickel plating, or a glaze, were both found to be incompatible with a 4% solution of hydrogen peroxide and peroxyacetic acid, after being exposed to the solution for two weeks at 160.degree. F.
In addition, it was found that various materials used to construct the transducer housing and assembly experienced deterioration after being subjected to a simulated long-term exposure to an acid solution of hydrogen peroxide and peroxyacetic acid. This was observed with an accelerated aging test. This test consisted of placing components constructed of various material types in vessels containing the hydrogen peroxide and peroxyacetic acid solution and subjecting them to increased temperature over a course of time. Without being limited to the theory, this test is based on the theory recognized in the art that at higher temperatures chemical or physical reactions will proceed faster due to the increased probability that two molecules will collide and chemically react.
Without being limited to a mechanism, method, or chemical, it is believed that chemically reactive liquids are necessary in sterilization processes to contact contaminants including but not limited to toxins, bacteria, virus, fungus, and spores (both fungal and bacterial), prions or protein structures, within a target area(s) either killing or neutralizing the bacteria, virus, fungus, and spores, or rendering the toxin, virus, or protein structure incapable of replication or otherwise interfering with the target's cellular physiology, or destroying or neutralizing the toxin. These chemically reactive liquids may be provided as an aerosol. For example, U.S. Pat. No. 4,512,951, which is incorporated herein by reference in its entirety, including any references cited therein, teaches using hydrogen peroxide to sterilize medical articles by condensing hydrogen peroxide-water vapors to deposit a film of liquid on the medical devices. The liquid film is then evaporated off the medical devices.
While the prior art attempted to coat the transducer, there were problems with these coatings. U.S. Pat. Nos. 3,729,138; 4,109,863; and 4,976,259, each of which is incorporated herein by reference in its entirety, including any references cited therein, teach that the optimum thickness of a glass barrier, which may be used as a protective plate and/or cover, on a transducer should be any multiple of one-half (½) the wavelength of the transmitted pressure (energy). The thicknesses of protective barriers have been calculated using wave transmission theories and their respective mathematical formulas known to those skilled in the art. It is estimated that roughly twenty percent (20%) of the energy emitted from the transducers is being transmitted into the liquid beyond the protective barrier. The prior art does not include techniques for further increasing the energy emitted from the transducer with a protective plate and/or cover.
U.S. Pat. Nos. 3,433,461; 3,729,138; 4,109,863; and 4,976,259, each of which is incorporated herein by reference in its entirety, including any references cited therein, teach that an effective thickness of a protective barrier material “interfaced with” a transducer can be any multiple of one-half (½) the wavelength of the transmitted pressure (energy) from the transducer. Prior art has taught that one-half (½) wavelength thick barriers constructed from non-conductive and/or insulating type materials like glass, could be effectively coupled with an ultrasonic transducer for generating aerosol, as long as they included a special design consideration for cooling the transducer, or the transducer was separated from the glass barrier with a layer of liquid. U.S. Pat. No. 3,433,461 teaches utilizing a 1.5 inch diameter transducer bonded to a metal barrier that is a one-half wavelength thick. A problem associated with using metal barriers is corrosion, which was acknowledged in U.S. Pat. No. 3,729,138. In addition, U.S. Pat. No. 3,433,461 discloses that heat has a detrimental effect associated with the operation of a transducer and that a metal barrier interfaced with a transducer permitted the use of much higher driving powers than in prior art devices, since it provided more heat dissipation. Further, the driving power supplied to the transducers is limited by the heat dissipation in the device, which is a function, in each case, of the total area of the generator.
According to U.S. Pat. No. 4,976,259, an attempt was made to bond a glass barrier to a piezoelectric crystal with an adhesive, but such an attempt did not improve on the prior art and resulted in a major loss of acoustic coupling of the ultrasonic energy into the glass cover as the adhesive bond deteriorated. The deterioration was due to high localized temperatures caused by reflected energy resulting from mismatched acoustical impedances.
The prior art does not currently include commercially effective techniques for constructing and operating a high frequency and high power aerosol producing transducer assembly consisting of one or more transducers bonded or adhered to a protective barrier constructed from non-metallic and/or insulative type materials, such as glass, with a thickness that is not one-half (½) of a wavelength. Furthermore, the prior art does not currently include high frequency and high power aerosol producing glass barrier and transducer assemblies that are capable of operating without additional liquid layers or liquid cooling means incorporated into the transducer assembly design.
Therefore, the need for a protective barrier for the aerosol producing transducer that is highly resistant to degradation caused by chemically reactive solutions exists. The protective barrier should withstand the heat generated by a transducer and should function effectively with the transducer to produce a fine aerosol at high output levels (which requires high energy emitted by the transducer). This heat is due to the high frequency and energy that is needed to achieve a high output of aerosolized liquid per hour with the aerosol droplets being less than about 10 microns in size. In general, within the effective frequency band, the higher the power at the effective aerosol producing frequencies, the larger the quantity of aerosol produced; and the higher the effective frequency the smaller the droplet size in the aerosol.